Algae
Volume 22(1): 31-36, 2007
[Note]
Unusual Rhizoidal Development in Bangia (Bangiales, Rhodophyta) –
Another Form of Vegetative Reproduction?
Christian Boedeker1*, Tracy J. Farr2 and Wendy A. Nelson2
1
National Herbarium of the Netherlands, Leiden University Branch, P.O. Box 9514, 2300 RA Leiden, The Netherlands
2
National Institute of Water and Atmospheric Research, Private Bag 14-901, Kilbirnie, Wellington, New Zealand
The gametophytic filaments of two genetically distinct taxa of Bangia from New Zealand showed unusual rhizoidal development in comparative culture experiments. In the past Bangia has been reported to possess simple,
colourless rhizoids that extend from the basal cells of the unbranched filaments, whereas in this study the rhizoids
observed became pigmented and multicellular. A reversal of growth direction occurred and filamentous extensions
developed from the rhizoids under some culture conditions. These extensions were either prostrate or resembled
new gametophytic filaments. This is the first report for filamentous members of the Bangiales of the development of
such stolon-like rhizoids, apparently serving as a form of vegetative reproduction.
Key Words: Bangia, perennial phase, stolon-like rhizoids, vegetative reproduction
Filamentous algae currently placed in the genus Bangia
Lyngbye (Bangiales, Rhodophyta) are found worldwide
on hard substrates in marine habitats in the upper intertidal zone (Lüning 1990; Brodie and Irvine 2003; Broom
et al. 2004). The macroscopic gametophytic phase consists
of superficially simple, unbranched, uniseriate filaments
(Fig. 1) that can become multiseriate by radial cell divisions at maturity (Figs 2, 3). Members of the Bangiales
have heteromorphic sexual life histories involving a filamentous (the recently erected freshwater genus
Bangiadulcis (Nelson 2007), Bangia, Dione, Minerva,
Pseudobangia), or leafy (Porphyra) gametophyte and a
microscopic sporophyte called conchocelis phase (Figs 4,
6) (Drew 1956; Garbary et al. 1980). The gametophytic
phase of both genera can reproduce asexually through
neutral spores/archaeospores released by vegetative
cells of the thallus (terminology follows Nelson et al.
1999).
In Bangia the first cell division of a haploid spore
results in two daughter cells, one of which becomes the
basal cell. All cell divisions in Bangia spp. are intercalary:
a uniseriate filament develops initially and the modified
basal cell does not undergo any further cell divisions
(Sommerfeld and Nichols 1970). Only the upper part of
the basal cell is pigmented and a protrusion of the lower
*Corresponding author ([email protected])
part forms the primary rhizoid, attaching the filament to
the substratum (Geesink 1973). These rhizoids are
colourless and show no internal structures. Secondary
rhizoidal outgrowths may grow intramatrically from
additional lowermost cells within a filament (Kylin 1921;
Smith 1969), and can lead to a branched appearance of
the holdfast when penetrating the surrounding sheath
(Sommerfeld and Nichols 1970). The development of secondary rhizoids from intercalary cells also has been
observed (Gargiulo et al. 2001).
In this study the unusual stolon-like development of
the rhizoids of two genetically distinct taxa of Bangia
from New Zealand is reported. Field material was collected in marine intertidal habitats on rocky shores on
the west coast of the South Island of New Zealand in
2000 for each of the two taxa Bangia sp. BMW (culture
BCB2, S41°46.47’, E171°11.63’) and Bangia sp. BGA (cultures B1HH and BCS, S42°26.31’, E171°11.63’ and
S41°35’, E171°53.7’, respectively). Unialgal stock cultures
were established in the algal culture collection at the
Museum of New Zealand Te Papa Tongarewa (WELT,
Holmgren et al. 1990), now transferred to the National
Institute of Water and Atmospheric Research,
Wellington, New Zealand. Collections were identified as
Bangia in the field, and DNA sequencing was carried out
as part of a larger project examining diversity in
Bangiales. From the results of nSSU sequencing, strains
32 Algae Vol. 22(1), 2007
Fig. 1. Filament of Bangia sp. BMW displaying normal rhizoidal development (cultured for 14 days at 12°C, 12:12 h L:D, 34 psu).
Fig. 2. Comparison of uni- and multiseriate filaments (B. sp. BFK, see Broom et al. 2003 for details).
Fig. 3. Female multiseriate filament consisting of zygotosporangia (B. sp. BFK).
Fig. 4. Sporophytic conchocelis phase (B. sp. BTS, see Broom et al. 2003 for details) developing from a phyllospore (arrow).
Fig. 5. Rectangular branching pattern of sterile conchocelis filaments of B. sp. BGA after 77 days at 12°C, 12:12 h L:D, 34 psu.
Fig. 6. Conchocelis filaments of B. sp. BMW with conchosporangial branches (arrows) after 70 days at 12°C, 12:12 h L:D and another
21 days at 10°C, 9:15 h L:D, 34 psu.
Boedeker et al.: Unusual Rhizoids in Bangia 33
were assigned to Bangia taxa as described in Broom et al.
(2004, see also for the corresponding GenBank accession
numbers). Stock cultures of the taxa were maintained at
12°C, 12:12 h light:darkness (L:D), 10-50 µmol photons
m–2 s–1, 34 psu.
Gametophytic filaments were grown from neutral
spores/archaeospores released from filaments in culture
after drying overnight and subsequent submersion.
About 40 spores per treatment for each taxon were transplanted to cover slips, allowed to settle overnight, and
then placed in petri dishes with f/2 growth medium
(Guillard and Ryther 1962). The growth of the developing filaments was investigated under five different culture conditions in laboratory experiments. Conditions
were: (1) 10°C, 9:15 h L:D, 34 psu; (2) 12°C, 12:12 h L:D,
34 psu; (3) 15°C, 15:9 h L:D, 34 psu; (4) 20°C, 16:8 h L:D,
34 psu; (5) 12°C, 12:12 h L:D, 5 psu. Photon flux densities
in all treatments were ca. 35 µmol photons m–2 s–1. The
development of the filaments was followed for 28 days.
On days 1, 3, 7, 10, 14, 21 and 28 of the experiments the
filaments were examined and measured using an
Olympus CK2 inverted microscope and digital micrographs were taken of each filament with an Olympus
DP10 digital camera (Olympus America Inc.).
A reversal of growth direction was observed in the filaments of the two Bangia taxa, with new growth occurring in the rhizoidal parts. In addition to extending apically due to intercalary cell divisions in the main filament, the rhizoidal part started extending in the opposite
direction. The rhizoids were extending and became pigmented under some conditions (Fig. 7). After approximately 14 days, cell walls were visible in pigmented
parts of the rhizoids of both taxa (Figs 8-10). These rhizoids continued to extend from the original filaments,
and showed further cell divisions. After ca. 28 days some
of the rhizoids showed stolon-like outgrowths resembling new gametophytic filaments (Figs 11, 12). One
taxon (BMW) showed this unusual development of rhizoids only under the low salinity regime of condition 5 (5
psu), while the other taxon (BGA) displayed stolon-like
rhizoids under conditions 2-4, with a higher percentage
of filaments displaying stolon-like rhizoids at higher
temperatures. After 28 days all filaments of BGA cultured at 20°C (condition 4) and all filaments of BMW at 5
psu (condition 5) had developed rhizoids with filamentous outgrowths.
The observed rhizoidal extensions have a lumpy
appearance with an irregular outline. These rhizoids are
5-15 µm in diameter and have a thin cell wall (< 1 µm).
The shape and length of cellular compartmentation in
the rhizoidal parts is very variable, ranging from rounded or triangular cells (2-10 µm wide) to stretched rhizoidal parts (many times longer than broad). The extent
of overall pigmentation in the rhizoids increased gradually with time, with short cells in the stolon-like parts
being dark and densely pigmented while the the long
rhizoidal parts were irregularly pigmented with
stretched strands of connected cytoplasm.
This is the first time that stolon-like development of
rhizoids of Bangia has been documented. The pigmentation, cell divisions and outgrowths of the rhizoids suggest a role beyond merely attaching filaments to the substrate; the stolon-like rhizoids and associated filaments
produced in this way may contribute to population
maintenance and growth through vegetative reproduction. In studies of other members of the Bangiales, various observations of asexual reproduction have been
recorded. Dixon and Richardson (1969) reported “perennating basal fragments” of Porphyra perforata J. Agardh
and Bangia at a site where the gametophyte phase was
absent and no trace of the microscopic sporophyte phase
had been detected. These perennating structures were
described as pigmented cells which released vegetative
cells that grew into new gametophytic thalli (loc. cit.). To
our knowledge, similar stolon-like development of the
rhizoids is not known from other red algae. The propagating rhizoids of the ulvophyte Blidingia minima var.
stolonifera differ by being colourless and containing no
nuclei or chloroplasts (Garbary and Tam 1989). Gargiulo
et al. (2001) documented resting cells in both the macroscopic thalli and microscopic conchocelis stage of freshwater Bangiadulcis (as Bangia atropurpurea), as well as in
situ germination of spores released by the conchocelis
giving rise to new gametophytic filaments. We have
observed both in situ germination of spores within Bangia
filaments, and the development of conchocelis filaments
in culture. Both of these clearly differ from the rhizoidal
growth we report here. Furthermore, the conchocelis
stage of the two Bangia taxa used here shows a very different overall morphology (Figs 4-6) from the described
stolon-like rhizoids. Conchocelis filaments have relatively long and regular cells, more or less uniform in diameter in sterile branches, branched at right angles and having one to several ribbon-shaped parietal chloroplasts.
The stolon-like rhizoids reported here may function as
perennial structures during unfavourable conditions:
their prevalence at high temperature and/or low salinity
indicates a taxon-specific response to stress, since the
34 Algae Vol. 22(1), 2007
Fig. 7. Pigmented rhizoid (arrow) of Bangia sp. BMW after 14 days at 12°C, 12:12 h L:D, 5 psu.
Fig. 8. Visible cell walls (arrow) within rhizoid of Bangia sp. BMW after 14 days at 12°C, 12:12 h L:D, 5 psu.
Fig. 9. Clearly visible cells and cell walls (arrow) within rhizoid of Bangia sp. BGA after 21 days at 15°C , 15:9 h L:D, 34 psu.
Fig. 10. Pigmented rhizoid of Bangia sp. BMW with visible cell walls and beginning outgrowth (arrow) after 21 days at 12°C, 12:12 h
L:D, 5 psu.
Fig. 11. New filamentous outgrowth (arrow) developing from a stolon-like rhizoid of Bangia sp. BGA after 27 days at 20°C, 16:8 h
L:D, 34 psu.
Fig. 12. Enlargement of same filament.
bc = basal cell, of = original filament.
Boedeker et al.: Unusual Rhizoids in Bangia 35
original material of both taxa had been acclimatised to a
temperature of 12°C and full marine growth medium (34
psu). In our experiments the development of filaments
extending from the rhizoids was observed early in the
experiment (i.e. from day 7-10). Five other taxa of Bangia
that were investigated (results not presented) did not
show similar rhizoidal development under any of the
described conditions.
There is no direct link between the geographical distribution of the taxa and the culture conditions under
which the stolon-like rhizoids were observed, but the
simulated conditions would at least sometimes occur in
the natural habitats. Bangia sp. BMW only formed the
described rhizoids at 5 psu and is often found in sites
with lowered salinity, such as freshwater streams influenced by the tides. Bangia sp. BGA showed a higher percentage of filaments with rhizoidal extensions at higher
temperatures. This taxon occurs further north than
BMW, experiencing summer surface temperatures of
20°C or more in the north of the North Island.
Broom et al. (2004) examined diversity within the filamentous members of the Bangiales and reported on distinct lineages which they considered warranted consideration as new genera. Subsequently 3 new taxa have been
described for filamentous Bangiales (Nelson et al. 2005,
Müller et al. 2005). The taxa considered in this paper
(BGA and BMW) both belong to the genus Bangia and
fall into separate clades in a Bangiales phylogeny as discussed in Broom et al. (2004), possibly representing two
separate species. The propagation of the rhizoids as
described here is not a synapomorphic character, but
seems to occurr independently in distant parts of phylogenetic trees.
Asexual reproduction plays an effective role in maintaining Bangia populations; some are known to be solely
asexual (Sheath and Cole 1980; Sheath et al. 1985).
Conditions suboptimal for reproductive development
may have triggered the rhizoid development in our
experiments. Members of the Bangiales possess a range
of sexual and asexual reproductive methods, and this
capacity is regarded as a major contributor to the success
of this widely distributed family (Cole and Conway
1980). The observations reported here suggest a further
vegetative mechanism that may be contributing to the
perennation of populations of some taxa in this order.
ACKNOWLEDGEMENTS
Field work, isolation and maintenance of culture col-
lections, and molecular sequencing to confirm the identity of taxa was funded by the New Zealand Foundation
for Research, Science and Technology, Project
MNZX0202. Judy Broom (University of Otago) is
thanked for sequencing.
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Received 10 February 2007
Accepted 10 March 2007